In U.S. Pat. No. 4,415,356 (the '356 patent) (corresponding to Canadian patent application No. 389,129) there is disclosed a process for autogenous oxygen smelting of sulfide materials containing base metals. The extensive prior art relating to autogenous smelting of copper and nickel sulfide materials is discussed in the '356 patent and the invention described therein is disclosed as:
"The invention is based on the discovery that in the oxidation smelting the matte grade generated in the smelting furnace can be controlled by dividing the metal sulfide material stream to be smelted such that a portion of the stream is subjected to at least partial or even dead roasting, is then mixed with additional fresh metal sulfide material before being fed to the flash smelting furnace along with flux in the usual manner. This technique permits an upgrading in the matte grade produced, and is particularly applicable to oxygen flash smelting." PA1 "It will be appreciated that the roasting step which forms part of the invention may be accomplished in equipment such as a fluid bed roaster. When this is done, a gas containing at least 10% of sulfur dioxide is produced which may be employed as feed for a sulfuric acid plant. In this way sulfur removed from the portion of concentrate which is roasted can be recovered and is not discharged to the atmosphere. Roasting in the fluid bed can be accomplished using air as the oxidant. PA1 The blend of roasted and dry unroasted concentrate, mixed with silicious flux, is injected into the smelting furnace in a stream of oxygen. The desired composition of matte to be obtained can be controlled by adjusting the ratio of calcine to green sulfide material in the feed. For a given concentrate, heat balance calculations will dictate the relative proportions of calcine and green sulfide material which have to be fed to yield the desired product on autogenous smelting." PA1 "It is preferred to dead roast only a proportion of concentrate fed to the smelter since in the way materials handling is minimized. Similarly, other sulfide materials equivalent in general metallurgical characteristics to sulfide concentrates, e.g., furnace mattes, can be treated in accordance with precepts of the invention. As noted hereinbefore, for a given sulfide material and a given furnace a sufficient amount of oxygen per unit weight of sulfides must be provided to supply the heat balance of the operation. Thus, for a given sulfide material, heat balance calculations will establish the relative proportions of calcined and uncalcined material to be employed, matte grade, or whether the given sulfide material is treatable by oxidation smelting. It will be apparent from the foregoing description that oxidation smelting, e.g., autogenous oxygen flash smelting, can be carried out in two stages. Thus copper concentrate can be flash smelted in a first operation to matte grade of about 55% while producing a slag which can be discarded; the matte can be granulated, ground and smelted in a second flash melter to yield white metal or blister copper with the slag from the second flash smelter being returned to the first smelter operation. Alternatively, the slag from the second operation can be slow cooled, concentrated and the concentrate returned. Calcine can be fed to either or both of the flash smelting operations along with the sulfide feed in accordance with heat balance requirements and to control product grade therefrom."
The '356 patent goes further to state:
Thus, the '356 patent discloses a process in which sulfur dioxide is a product of the roasting step and that silicious flux is mixed with the blend of roasted and unroasted concentrate and injected into the smelting furnace. The '356 patent also considers possible variations in the disclosed process in the following language:
In gaining experience with the process of the '356 patent, applicant has found that the silica-based slags used in the patented process require a difficult slag cleaning operation in an electric furnace or slow cooling and flotation of copper metal to achieve good cooper recovery. In addition, when blister copper is produced from iron-containing materials, the silica-based slags are viscous and contain high magnetite concentrations.